In members in the automotive field, household appliance field, and construction material field, surface treated steel sheet which imparts corrosion prevention is being used. In particular, hot dip galvanized steel sheet which can be inexpensively produced and which is excellent in corrosion prevention is being used.
In general, hot dip galvanized steel sheet is produced by the following method using a continuous hot dip galvanization facility. First, a slab is hot rolled, cold rolled, and heat treated to obtain a thin-gauge steel sheet. This is degreased and/or pickled by a pretreatment step for the purpose of cleaning the surface of the base material steel sheet or, omitting the pretreatment step, is heated in a preheating furnace to burn off the oil on the surface of the base material steel sheet surface, then is heated to recrystallize and anneal it. The atmosphere at the time of recrystallization and annealing is an Fe reducing atmosphere since at the time of the later plating treatment, Fe oxides would obstruct the wettability of the plating layer and the base material steel sheet or the adhesion of the plating layer and base material steel sheet. After the recrystallization and annealing, without contacting the air, the steel sheet is continuously cooled to a temperature suitable for plating in an Fe reducing atmosphere and dipped in a hot dip galvanization bath for hot dip galvanization.
In a continuous hot dip galvanization facility, the types of heating furnaces which perform the recrystallization and annealing include DFF (direct flame furnaces), NOF (nonoxidizing furnaces), all radiant tube type (all reducing) types or combinations of the same etc., but for ease of operation, less roll pickup in the heating furnace, the ability to produce high quality plated steel sheet at a lower cost, and other reasons, the mainstream practice has been to make the entire inside of the furnace an Fe reducing atmosphere and make the heating furnace an all radiant tube type. The “roll pickup” referred to here means the deposition of oxides or foreign matter from the surface of the steel sheet on the rolls in the furnace at the time of running through the furnace. After deposition, defects in appearance occur at the steel sheet, so this has a detrimental effect on quality and productivity.
In recent years, in particular in the automotive field, to achieve both the function of protecting the passengers at the time of collision and lighter weight aimed at improvement of the fuel efficiency, use of hot dip galvanized steel sheet which is made higher in strength of the base material steel sheet by inclusion of elements such as Si and Mn has been increasing.
However, Si and Mn are elements which are more easily oxidizable compared with Fe, so at the time of heating in recrystallization and annealing in the all radiant tube type of furnace, even in a reducing atmosphere of Fe, Si and Mn end up oxidizing. For this reason, in a steel sheet which contains Si and Mn, in the process of recrystallization and annealing, the Si and Mn present in the steel sheet surface oxidize. Further, the Si and Mn which thermally diffuse from the inside of the steel sheet oxidize at the steel sheet surface whereby gradually the Si and Mn oxides become concentrated. If the Si and Mn oxides concentrate at the steel sheet surface, in the process of dipping the steel sheet in the hot dip galvanization bath, contact between the molten zinc and steel sheet would be obstructed, which would cause a drop in the wettability of the plating layer and adhesion of the plating layer. If the plating layer falls in wettability, nonplating defects occur and result in defects in appearance and/or defects in corrosion prevention. If the plating adhesion falls, when press forming this plated steel sheet, peeling of the plating occurs and results in defects in appearance and/or defects in corrosion prevention after forming, so becomes a major problem.
As the art for suppressing concentration of oxides of Si and Mn, as art focusing on the recrystallization and annealing process, PLT 1 shows oxidizing the steel sheet surface so that the thickness of the oxide film becomes 400 to 10000 Å, then reducing the Fe in the furnace atmosphere containing hydrogen and then plating. Further, PLT 2 shows the method of oxidizing the steel sheet surface and controlling the oxygen potential in the reducing furnace to thereby reduce the Fe and internally oxidize the Si so as to suppress the concentration of Si oxides at the surface, then plating. However, in these art, if the reduction time is too long, Si concentrates at the surface, while if too short, an Fe oxide film remains on the steel sheet surface. In the actual case where the oxide film on the steel sheet surface becomes uneven in thickness, there is the problem that adjustment of the reducing time is extremely difficult and issues in the plating layer wettability and plating layer adhesion are insufficiently resolved. Furthermore, if the Fe oxide film of the surface at the time of oxidation becomes too thick, there is the problem that roll pickup is caused.
PLT 3 solves the above problem which was due to causing Fe to oxidize once, has as its object to suppress the concentration of the Si and Mn oxides, and shows a method comprising lowering the oxygen potential (log(PH2O/PH2)) of the atmosphere in the recrystallization and annealing in an all radiant tube type of furnace to a value at which Fe and Si and Mn will not be oxidized (be reduced), then plating. However, in this art, to reduce Si and Mn, it is necessary to greatly lower the steam concentration of the atmosphere or greatly raise the hydrogen concentration, but there is the problem that this is poor in industrial practicality and also the problem that the Si and Mn which remain at the steel sheet surface without being oxidized obstruct the reaction between the plating and base material steel sheet and, further, react with the oxides floating on the surface of the bath to form Si and Mn oxides at the time of dipping in the plating bath, so the plating wettability and plating adhesion fall.
PLT 4 shows a method of raising the oxygen potential in the atmosphere in the recrystallization and annealing in an all radiant tube type of furnace until Si and Mn internally oxidize, then plating. Further, PLTs 5 and 6 show methods of carefully controlling the means and conditions for raising the oxygen potential to suppress the surface concentration of both Fe oxides and Si and Mn oxides, then plating. However, if raising the oxygen potential, Si and Mn internally oxidize, but Fe oxidizes. On the other hand, with an increase of oxygen potential of an extent where Fe does not oxidize, the internal oxidation of Si and Mn becomes insufficient and Si and Mn oxides concentrate at the surface. In the arts of adjusting the oxygen potential of the atmosphere which are described in PLTs 4 to 6, there is the problem that the issues in plating layer wettability and the plating layer adhesion are not sufficiently resolved.
Furthermore, as art for suppressing the concentration of Si and Mn oxides, as the above-mentioned means increasing the steps of production of the general continuous type hot dip galvanization, PLT 7 shows the method of performing annealing two times, pickling and removing the surface concentrates of Si which are formed on the surface after the first annealing so as to suppress the formation of surface concentrates at the time of the second annealing, then plating. However, when the Si concentration is high, pickling is not enough to completely remove the surface concentrates, so the plating wettability and plating adhesion are insufficiently improved. Further, facilities for two annealing operations and pickling facilities are newly required for removing the surface concentrates of Si, so there is the problem of an increase in the capital costs and production costs.
PLTs 8 and 9 show methods of preplating the steel sheet surface by Cr, Ni, Fe, etc. before or after recrystallization and annealing, then plating. However, in these art, there are the problem that when preplating before recrystallization and annealing, the heating at the time of annealing causes the preplated elements to diffuse in the steel sheet and the steel sheet to fall in strength and elongation and the problem that the Fe or Si and Mn which diffuse at the steel sheet surface oxidizes. Further, when preplating after recrystallization and annealing, oxides are formed on the steel sheet surface, so there is the problem that preplating unevenly deposits on the steel sheet and has difficulty covering the concentrated oxides. Further, this method has the problem that no matter whether performing the preplating before or after the recrystallization and annealing, cost is incurred in the materials of the preplating or costs are incurred in the preplating facilities, so the increase in steps leads to an increase in the production costs.
Furthermore, in art which suppresses the concentration of the Si and Mn, as art which focuses on causing internal oxidation in advance in the hot rolling step, PLT 10 shows the art of controlling the oxygen potential in the hot rolling step so as to cause internal oxidation of Si and using the resultant thin-gauge steel sheet to produce hot dip galvanized steel sheet by a continuous hot dip galvanization facility. However, in this art, at the time of the cold rolling step and other rolling, the layer of internal oxidation also ends up being rolled together, so the internal oxidation layer becomes smaller in thickness and Si oxides end up concentrating at the surface in the recrystallization and annealing process, so there is the problem that the plating wettability and plating adhesion are insufficiently improved. Further, there is the problem that if causing internal oxidation in the hot rolling step, the simultaneously formed Fe oxides cause roll pickup.
PLT 11 shows the method of controlling the oxygen potential in the atmosphere in the heating furnace and the oxygen potential in the atmosphere at the topmost part of the soaking furnace high in the same way and controlling the oxygen potential of the top part of the soaking furnace to be higher than the oxygen potential at the bottom part of the furnace by a certain degree to plate the high-Si containing steel sheet. However, by this method as well, the plating adhesion is insufficient.